Yellow crookneck is pepo and butternut is moschata. Pepo and moschata can indeed cross pollinate according to some sources, but will not according to others. But I have seen more sources state that they will indeed cross pollinate.
It could also depend on your blooming time if they crossed or not- if they were blooming at the same time, that upps the likelihood of crossing, but if they really bloomed at different times, that could lessen the crossing possibility.
I'm still a bit confused too about curbit crossing, but I've been trying to learn because it's one of my favorite things to grow! But very much yes, cross pollination will have an effect on the seeds produced. This years crop will be what you expect, but the seeds might produce something entirely different.

So the seed might not be viable, or it might yield up a crossbreed. Not so odd if you cross two summer or two winter squashes, but I have no idea what you might end up with crossing a summer and winter squash. Long ago I had an acorn squash and cucumber cross, and the result was viable seed that produced what looked like an acorn squash but tasted like a weird cucumber.

Personally, I would probably save the seed and grow it next year and see what I accidentally made. You might get something great or gross.

How to know when the seeds are dry enough? That's a tough call, it depends on how you are drying them, what the humidity is... I usually leave my seeds out for a couple weeks before storing them, I let the shells on them get dry and hard while the "meat" inside is still tender and flexible. I also do a germination test on them before storing them.